HSS Roof Purlins Connection

[Szubert80]

Hi, I have this small canopy steel frame (please see attached pdf) showing roof purlins connected with shear tabs w/ (2) 5/8 bolts each end.
I have seen these type of connections before but I don`t think it is a good practice to have the bolts in this configuration.
My main concern is that bolts would experience considerable tension due to prying action. Please let me know what do you think about this.

 

[KootK]

- I'd be inclined not to worry about this for something where the loads will be pretty nominal.

- I agree, its not the most direct configuration for transfer of wind loads etc. Mechanically, knife plates oriented perpendicular to the roof surface or clip angles and through bolts would be preferable. There may be erection advantages with this setup however.

- Any prying can probably be addressed by making your plates thick enough to mitigate prying.

 

[CANPRO]

Is there a reason you can't just turn the connection 90 degrees?

When you say small canopy, how small do you mean? Depending on the total width and the site location relative to the fabricator, I'd be inclined to shop weld the purlins to the sloping HSS beams and send the roof as an assembly - saves 6 connections on site, which reduces crane time, and that method also reduces shop labour and material.

 

[Szubert80]

KootK & CANPRO - thank you for your feedback.

I have already suggested fabricator to make it into one frame and I`m waiting for his response.

The foot print is only 4.0m x 3.0m but the snow load is 3.6kPa in this area.

They want to have u/s of purlins to be flush with HSS beams. They are only 3x3 HSS and it is hard to fit 2 bolts in vertical for this connection.

Also the weld between shear tab and HSS beam need to be butt weld rather than fillet due to torsion.

 

[MIStructE_IRE]

You could simply calculate the forces and will probably find it still works fine. I had something similar on a curtain wall support where I had to get your detail to work to suit architectural detailing. I followed the load paths through and it worked fine - against my original intuition however I’ll admit.

If possible though it would be far better to rotate by 90 deg.

 

[BUGGAR]

How are you considering the welds from the plate to the HSS for rotating about their longitudinal axis along the HSS? Maybe this is just a shipping/erection stress?

 

[Agent666]

If the width for transportation allowed, I'd go one further than CANPRO and look at just welding up the entire top bit in the shop and plonk it on 4 columns on site (probably weld it as well on site with some backing plate tacked into the inside of the top of the column to guide it all into place during erection). But you could easily come up with an L shaped cap plate with through bolts or hollo-bolts to that bottom ring.

That's a lot of ugly as sin bolted connections to be exposed in something that presumably has some architectural merit. My opinion obviously, but the inner architect in me is screaming about it in my ear like Jim Carey from dumb and dumber.

 

[Agent666]

If you must do it that way, you can make the cleats with a stiffener on both the rafter and the purlin (so cleat is a Tee shape with a bolt either side of the stiffener), that way its more resting on a stiffened ledge than relying on the bending of just the plate about its weak axis.

 

[Agent666]

Whats wrong with a horizontal end plate with a bolt or hollo-bolt either side of the purlin as another option to throw out there. Thinking about it, I'm not a great fan of the end connections being like that on the rafter supporting the purlins, has no torsional stiffness, kind of wants to twist the slotted plate due to the eccentric purlin load. End plate on the purlins would address this as would prevent the twist.

 

[MotorCity]

Not seeing the prying action. To me, it looks like the HSS will be supported by (sit on) the gusset which will resist any loads in the axial direction of the bolts. There will only be shear in the bolts as the HSS tries to "slide off" the gusset.

Also, if this is located outdoors, I would avoid the slotted connection due to the difficulty of inspecting for corrosion.

 

[CANPRO]

Agent666, I'm a fan of pre-fabricating as much as possible and I had a similar thought to what you suggested - only fall back is splicing those HSS columns can be a pain if you have to keep them hidden within limited architectural space (which is often the case).

I still don't like the horizontal plate, I think I'd prefer the connection shown below for the typical purlin.